Sucker rod

ABSTRACT

A sucker rod has end fittings secured to a fiberglass rod by threading a portion of the fiberglass rod into an insert assembled in the rod cavity of the end fitting.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

BACKGROUND OF THE INVENTION

The present invention relates to a sucker rod for a well pump and to a method and apparatus for attaching an end fitting to a sucker rod.

During production of a well, such as an oil well, the pressure from the reservoir may become insufficient to force the fluid to the surface. If so, a pump attached to the end of a sucker rod string may be lowered into the well. The upper end of the sucker rod string is typically attached to a pump jack or similar apparatus on the surface. The pump jack reciprocates the sucker rod string to alternately raise and lower a piston in the barrel of the pump which is submerged in the fluid in the well. When the piston is raised in the barrel by the sucker rod string, a check valve in the piston closes preventing fluid above piston from flowing back into the barrel and the lowering pressure in the barrel opens a check valve in the barrel allowing fluid in the well to flow into the barrel. When the piston is lowered in the barrel by reciprocation of the sucker rod string, the check valve in the barrel closes trapping fluid in the barrel and the check valve in the piston opens allowing the piston to move downward in the barrel by allowing the fluid in the barrel to flow past the piston where it will be trapped when the piston is raised on the next upstroke thereby raising the level of fluid in the well.

Steel sucker rods, typically about twenty five (25) feet in length and threaded at each end, have been joined end-to-end to make up sucker rod strings. However, steel is heavy and powerful equipment is required to reciprocate a steel sucker rod string. In addition, steel is subject to corrosion in the environment of a well and repair or replacement of failed steel sucker rod strings is expensive and difficult.

Fiberglass sucker rods were introduced in the 1970's. A fiberglass sucker rod comprises a fiberglass rod and an end fitting affixed to each end of the rod. Fiberglass sucker rods are typically 37.5 feet in length although 25 and 30 foot lengths and custom lengths are available. A fiberglass sucker rod weighs approximately one-third of the weight of an equal sized steel sucker rod making transportation, handling and installation significantly easier and less expensive and reducing the cost of the pump jack and the power necessary to reciprocate the sucker rod string.

The fiberglass rod, commonly available in diameters ranging from 0.625 inches to 1.25 inches, comprises long parallel strands of glass fiber in a plastic matrix. The fiberglass rod is typically formed by the pultrusion process where glass fiber is fed through a carding plate and then impregnated with a thermosetting resin such as vinyl ester, isothalic polyester or epoxy and preheated with a radio frequency preheater. The impregnated fiber is then pulled through a heated die which forms the final shape and size of the rod and cures the thermosetting resin.

The end fittings of a fiberglass sucker rod are typically made of steel and have external shapes and dimensions conforming to recommendations of the American Petroleum Institute (API). A cylindrical first portion of the elongate end fitting extending longitudinally from a first end of the fitting includes a surface (called a coupler) defining a screw thread enabling joining of the sucker rod to another sucker rod when making up a sucker rod string. The two end fittings of a sucker rod may have threads of opposite gender enabling an end fitting of one rod to be threaded directly into an end fitting of a second sucker rod or the end fittings may be the same gender requiring a coupling having threads of the opposite gender to join the end fittings of the sucker rods.

A second longitudinal portion extending from the first or threaded portion of the end fitting toward the second end of the fitting defines a square cross-section providing plural flat surfaces for engagement by a wrench enabling the application of torque to the fitting when making up the sucker rod string.

A third longitudinal portion extending from the second portion to the second end of the fitting has an annular cross-section with a cylindrical outer surface and an inner surface defining a rod cavity extending longitudinally in the end fitting from an aperture in the surface of the second end of the fitting. The rod cavity is typically circular in cross-section with a diameter which varies along the longitudinal axis of the fitting to define one or more substantially frustoconical cavity portions arrayed end-to-end along the longitudinal axis of the fitting with the larger diameter base of the frustrum at the end of the frustrum most remote from the second end of the fitting.

Typically, the end fittings are attached to the fiberglass rod with a thermosetting adhesive which adheres to the fiberglass rod and which hardens to form a wedge(s) in the frustoconical portion(s) of the rod cavity. To prevent the adhesive from adhering to the steel end fitting, the surface of the rod cavity is coated with a release agent which is cured. The adhesive resin, such as epoxy, is added to the rod cavity and the fiberglass rod is inserted into the cavity. Typically, the resin is cured by heating the sucker rod assembly for approximately one hour. After the adhesive resin has cured, tension is applied to the rod to set the adhesive wedges in the steel fitting. Since the adhesive resin is not adhered to the end fitting, the fitting is restrained to the rod by the bond between the adhesive and the rod and the physical interference between the wedge(s) of cured adhesive and the corresponding frustoconical surface portion(s) of the rod cavity.

As the sucker rod string is reciprocated, cyclic tension, compression and other forces may be exerted on the sucker rod. As a result of the angular orientation of the conical surface of the adhesive wedge to the longitudinal axis of the fiberglass rod, a component of the tension force on the sucker rod is exerted normal to the longitudinal axis of the fiberglass rod radially compressing the rod. In addition, the cyclic nature of the forces exerted on the sucker rod is believed to cause creep in the adhesive wedge further compressing the fiberglass rod radially near the intersection of the rod and the smaller diameter end of the conical wedge. Although the exact nature of the failure mode is unknown, fiberglass sucker rods commonly fail proximate the point at which the rod meets the end fitting where the radial compression of the rod is expected to be greatest. Since introduction of fiberglass sucker rods there have been continued efforts to improve the sucker rod with much of the effort directed to the relationship of the steel end fittings and the fiberglass rod and in particular to changing the shape of the interface between cured adhesive, the fiberglass rod and the end fitting.

What is desired, therefore, is a fiberglass sucker rod assembly which is stronger, has improved life and more consistent performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cutaway pictorial of a well pumping system.

FIG. 2 is an elevation drawing of an exemplary fiberglass sucker rod.

FIG. 3 is a section view of an end fitting for the fiberglass sucker rod of FIG. 2.

FIG. 4 is a section view of an end fitting secured to the fiberglass rod of FIG. 2.

FIG. 5 is a perspective drawing of an insert for securing the end fitting of FIG. 3 to a fiberglass rod.

FIG. 6 is a first plan view of an annular segment of the insert of FIG. 5.

FIG. 7 is an end view of the annular insert segment of FIG. 6.

FIG. 8 is an elevation view of the annular insert segment of FIG. 6.

FIG. 9 is a second plan view, opposing the first plan view, of the annular insert segment of FIG. 6.

FIG. 10 is a first plan view of a key segment of the insert of FIG. 5.

FIG. 11 is a section view of the key segment of FIG. 10 along line A-A.

FIG. 12 is an end view of the key segment of FIG. 10.

FIG. 13 is a second plan view, opposing the first plan view, of the key segment of FIG. 10.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring in detail to the drawings where similar parts are identified by like reference numerals, and, more particularly to FIG. 1 a well pumping system 20 comprises generally a down-hole pump 22 which is connected to a pump drive system or pump jack 24 at the surface by a sucker rod string 26 which is extends down a tubing lined wellbore 28. The exemplary pump drive system 20 comprises a walking beam 32 which is pivotally mounted on a Sampson post 34 and pivoted by a power unit 36 including a motor and a gearbox which rotates a crank 38 that is connected to the walking beam by a crank pin link 40. The sucker rod string 26 is typically connected to a polished rod 42 that passes through seals in a stuffing box 44 at the surface. The polished rod is connected to a horsehead 48 by a bridle 46 which converts the arcing motion at the end of the walking beam to a substantially vertical reciprocating motion. The sucker rod string 26 comprises multiple sucker rods 30 which are connected together end-to-end. End fittings 31 of the sucker rods 30 terminate in screw threads enabling plural sucker rods to be connected when making up the sucker rod string 26. The screw threads at the respective ends of the sucker rod may be of opposite gender enabling a sucker rod string to be assembled by threading the end fitting 31 of one sucker rod directly into the end fitting 31 of the next rod or both end fittings may have screw threads of the same gender enabling joining of adjacent sucker rod assemblies with a coupler having screw threads of the opposite gender. Since wells are commonly not straight, sucker rod guides 50 or centering accessories may be attached to the sucker rods of the sucker rod string to center the rods in the well tubing which lines the wellbore 28 and protect the sucker rods and the well tubing from abrasion.

A fiberglass sucker rod typically comprises a fiberglass rod having a diameter between 0.625 inches and 1.25 inches with a steel end fitting affixed to each end of the fiberglass rod. Assembled fiberglass sucker rods are typically 37.5 feet in length, although 25 and 30 foot sucker rods and custom length sucker rods are available. The steel end fittings are typically secured to the fiberglass rod by a thermosetting resin adhesive, such as epoxy. The end fitting includes a rod cavity that extends longitudinally in the fitting from the second end of the fitting, that is, the end of the fitting opposite of the threaded end. The rod cavity is typically circular in cross-section with a diameter that varies to define one or more frustoconical cavity portions spaced along the longitudinal axis of the fitting with the smaller base(s) of the frustrum(s) proximate the second end of the fitting. To avoid adherence of the adhesive resin to the steel end fitting, the surface of the rod cavity is coated with a release agent which is cured. The resin adhesive is added to the rod cavity and the fiberglass rod inserted into cavity. The rod assembly is heated for at least an hour to cure the adhesive which forms a rigid mass adhering to the fiberglass rod and conforming to the inner surface of the rod cavity. Physical interference between the frustoconical portion(s) of the cured adhesive and the rod cavity secures the adhesive mass in the fitting.

Fiberglass sucker rods commonly fail near the point where the fiberglass rod projects from the end fitting. The precise cause of failure is not known but is believed to be related to radial compression of the fiberglass rod which is greatest near where the rod projects from the fitting. As a result of the angular orientation of the outer surface of the adhesive mass relative to the longitudinal axis of the fiberglass rod, tensile forces applied to the sucker rod produce a radially compressive force on the fiberglass rod. In addition, the cyclic force applied to the reciprocating sucker road may cause creep in the plastic adhesive mass forcing the plastic between the rod and the wall of the rod cavity and increasing the radial compression of the rod at a point near where the rod exits the end fitting. Since fiberglass sucker rods were first introduced, efforts have been made to improve the strength and life of the sucker rods usually by changes in the shape and/or size of the rod cavity and the corresponding wedge of adhesive. The inventors concluded that substantial improvements in the strength, life and utility of fiberglass sucker rod assemblies would be possible by changing the mechanism used to secure the fiberglass rod to the end fittings.

Referring also to FIG. 2 an exemplary fiberglass sucker rod 60 of new construction includes a fiberglass rod 62 having an end fitting 52 affixed to the respective ends of the rod. The end fittings 52 of the exemplary sucker rod 60 preferably have external surfaces conforming to the recommendations of the American Petroleum Industry (API). Each elongate end fitting includes a threaded portion 64 proximate one end of the fitting which forms the respective ends 66, 68 of the sucker rod 60. Both end fittings 52 of the exemplary sucker rod 60 have threads of the same gender and a coupler with threads of the opposite gender would be used to engage the threaded portions of the fittings when connecting successive sucker rods to make up a sucker rod string. However, respective end fittings of sucker rods commonly have threads of opposite gender enabling sucker rod assemblies to be connected by directly threading together end fittings of opposite gender on two sucker rods. Distal of the end of the sucker rod assembly and the threaded portion 64 of the end fitting 52, each end fitting defines a wrench engagement portion 70 (indicated by a bracket) typically having a square cross-section and defining plural wrench flats 72 enabling the application of torque to the end fitting when screwing together plural sucker rods. Between the wrench engagement portion and the second ends 80 of the end fittings is a portion with a cylindrical outer surface 76.

Referring also FIG. 3, the end fitting 52 defines a rod cavity 90 extending longitudinally from an aperture 81 in the second end 80 of the fitting, that is, the end of the fitting opposite of the threaded portion 64. The rod cavity 90 is generally circular in cross-section and defines a generally frustoconical portion 92 (indicated by a bracket) extending along the longitudinal axis 94 of the fitting and having a smaller cross-section 96 proximate the second end 80 of the fitting and a larger cross-section 98 more distal of the second end of the fitting. Preferably, the rod cavity 90 defines plural generally frustoconical portions and, more preferably, three generally frustoconical portions 92, 100, 102 (indicated by brackets) spaced along the longitudinal axis 94 of the end fitting 52.

Referring also to FIGS. 4 and 5, to secure the fiberglass rod 62 of the exemplary sucker rod 60 to the respective end fittings 52 a threaded portion 104 of the rod is engaged with threads 106 defined by the inner surface 108 of a generally tubular insert 110 in the rod cavity 90 of the end fitting. The elongate insert 110 has an annular cross-section with screw threads 106 defined on the surface of the inner aperture 108. The outside diameter of the annular cross-section of the insert 110 varies along the length of the insert to form an exterior surface 112 that substantially conforms in size and shape to the rod cavity 90 including one or more frustoconical portions 160 each having a smaller diameter base 162 proximate a first end 101 of the insert which will be nearest to the second end 80 of the end fitting 52 when the insert is installed in the rod cavity and a larger diameter base 164 that is more distal of the first end of the insert.

The insert 110 comprises plural, preferably four, segments 120, 122, 124, 126 defined by two parallel planes 132, 134 extending the length of the insert on opposing sides of and, preferably, equal distance from the insert's longitudinal axis 130. Referring also to FIGS. 6-9, the parallel planes 132, 134 define a first annular segment 120 and a second annular segment 122 and referring also to FIGS. 10-13, a first key segment 124 and a second key segment 126. Each segment has an outer surface comprising an arc of the outer surface 112 of the insert 110 and an inner surface comprising an arc of the inner surface 108 of the insert. If the parallel planes 132, 134 are equally spaced from the longitudinal axis 130 of the insert 110, the first annular segment 120 and the second annular segment 122 will be substantially identical except for the longitudinal position of the portion of the screw threads 106 formed on the inner arc 140 of the respective segments. Likewise, the first 124 and second 126 key segments will be identical except that the screw threads 106 on the inner arc 125 will be shifted longitudinally to align with the screw threads on the first 120 and second 122 annular segments so that the lead of the screw threads is consistent around the surface 108 when the insert is assembled in the rod cavity. To enable assembly of the insert 110 in the rod cavity 90 of the end fitting 52, the position of the planes 132, 134 is chosen so that the maximum width 142 (indicated by a bracket) for the first and second annular segments 120, 122 is less than the minimum diameter of that portion of the rod cavity 90 into which the maximum width portions of the annular segments will be inserted. The segments 120, 122, 124, 126 of the insert comprise a plastic which may be reinforced, for example with glass fiber and which is preferably a thermoplastic, such as polyphenylene sulfide (PPS), polyether ether ketone (PEEK), polyetherimide (PEI), and PPA (polyphthalamide). Thermoplastic inserts can be quickly, accurately and relatively inexpensively formed by injection molding or a similar process.

To assemble the insert 110 in the rod cavity 90, one of the annular segments 120, 122 is aligned proximate the center of the rod cavity and inserted into the cavity and pushed against the wall of the cavity. A tab 144 projecting from the outer surfaces of each of the annular segments 120, 122 proximate the first end 101 is engaged with a radial groove 77 in the surface of the second end 80 of the end fitting to restrain the segment against rotation in the rod cavity when the fiberglass rod 62 is threaded into the insert 110. The remaining annular segment is inserted into the rod cavity 90 near the middle of the cavity and moved to the wall of the cavity at a location opposite of the first annular segment and with the tab 144 engaged with the radial groove 77 in the surface 80 of the end fitting 52. Then, the key segments 124. 126 are inserted in the cavity between the planar surfaces 146, 148 of the annular segments 120, 122 and pushed against the rod cavity's wall on opposite sides of the cavity. To complete the connection of the fiberglass rod 62 and the end fitting 52, a threaded portion of the fiberglass rod is engaged with the threads 106 on the inner surface 108 of the assembled insert 110 and the rod is screwed into the insert. The fiberglass rod 62 prevents the insert segments 122, 124, 126, 128 from moving away from the wall of the rod cavity 90 and the frustoconical portions 160 (indicated by brackets) of the outer surface 112 of the insert 110 engage the corresponding portions of the rod cavity's wall to prevent withdrawal of the rod and the insert from the end fitting. The screw threads on the fiberglass rod 62 and on the segments 120, 122, 124, 126 of the insert 110 resist reverse rotation of the rod.

The screw threads 104 on the fiberglass rod and insert spread the stress produced by the load on the sucker rod over a longer portion of the rod 62 and reducing localization of any radial compression of the rod increasing the strength and life of the fiberglass sucker rod. Although it is sufficient for assembly to provide threads on only the portion of the fiberglass rod 62 which engages the insert 110, preferably the threads are defined along the entire length of the fiberglass rod. Continuing the threads for the length of the fiberglass rod 62 reduces any concentration of stress resulting from any discontinuity at the juncture of the threaded and unthreaded portions of the rod. In addition, with threads defined along the full length of the rod, sucker rod guides 63 and other sucker rod accessories with an internal thread can be located at any desired position on the fiberglass rod by either threading the accessory onto the rod or by bolting a split accessory, with thread engaging surfaces on an internal surface, onto the rod.

Preferably, the fiberglass sucker rod 60 also includes a tubular plastic sheath 75 which encircles the rod to protect the rod and the well bore tubing from being abraded by the reciprocating sucker rod and in the event a highly loaded rod should break the sheath will contain any fiberglass fragments reducing well contamination and clean up.

Securing the end fittings of a fiberglass sucker rod to a threaded fiberglass rod with a threaded insert assembled in the rod cavity of the end fitting reduces the stress on the rod permitting use of a smaller diameter fiberglass rod, increases the life of the sucker rod, and reduces the time required to manufacture the sucker rod. In addition, in the event of a failure of the threaded fiberglass rod there is less likelihood of contaminating the well with fiberglass fragments and incurring expensive cleanup because the most likely mode of failure is stripping of the stripped threads inside the fitting which will retain the stripped threads.

The detailed description, above, sets forth numerous specific details to provide a thorough understanding of the present invention. However, those skilled in the art will appreciate that the present invention may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuitry have not been described in detail to avoid obscuring the present invention.

The terms and expressions that have been employed in the foregoing specification are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims that follow. 

I (we) claim:
 1. A sucker rod comprising: (a) an elongate end fitting including a portion defining a cavity extending longitudinally from an end portion of said fitting and having a substantially circular first cross-section of a first diameter proximate said end portion and a substantially circular second cross-section of a second diameter more remote from said end portion than said first cross-section, said second diameter being greater than said first diameter; (b) an insert comprising an assemblage of plural insert segments, said assemblage having: (i) a first end; and (ii) an annular cross-section defining an outer surface including a first portion proximate said first end having a diameter substantially the same as said first diameter of said cavity and a second portion more remote from said first end having a diameter substantially the same as said second diameter of said cavity and an inner surface defining a screw thread; and (c) an elongate rod having a portion of an outer surface defining a screw thread, said rod secured to said end fitting when said insert is assembled in said cavity of said end fitting and said screw thread of said rod is engaged with said screw thread of said insert.
 2. The sucker rod of claim 1 wherein said rod comprises fiberglass.
 3. The sucker rod of claim 1 wherein said portion of said outer surface defining said screw thread extends from a first end of said rod to a second end of said rod.
 4. The sucker rod of claim 1 wherein said assembly of insert segments comprises: (a) a first annular segment comprising a first portion of said outer surface of said insert and a first portion of said inner surface of said insert and defined by a plane extending longitudinally from said first end of said insert to a second end of said insert and spaced apart from a longitudinal axis of said insert; (b) a second annular segment comprising a second portion of said outer surface of said insert and a second portion of said inner surface of said insert and defined by a second plane parallel to said first plane and spaced apart from said first plane a distance greater than said first plane is spaced from said longitudinal axis of said insert, said second plane extending from said first end of said insert to said second end of said insert; (c) a first key segment comprising a third portion of said outer surface of said insert and a third portion of said inner surface of said insert extending between a first surface of said first annular segment defined by said first plane and a first surface of said second annular segment defined by said second plane; and (d) a second key segment comprising a fourth portion of said outer surface of said insert and a fourth portion of said inner surface of said insert extending between a second surface of said first annular segment defined by said first plane and a second surface of said second annular segment defined by said second plane.
 5. The sucker rod of claim 1 wherein a chord length defined by an intersection of said first plane with said outer surface of said first annular segment is shorter said first diameter of said cavity.
 6. The sucker rod of claim 1 further comprising: (a) an end surface of said end fitting defining a radial groove; and (b) a tab extending from said outer surface of said insert and engageable with said groove.
 7. The sucker rod of claim 1 wherein said plural insert segments comprise a thermoplastic.
 8. The sucker rod of claim 7 wherein said thermoplastic is reinforced.
 9. The sucker rod of claim 7 wherein said thermoplastic comprises one of polyphenylene sulfide, polyether ether ketone, polyetherimide and polyphthalamide.
 10. The sucker rod of claim 1 further comprising a sheath encircling a portion of said fiberglass rod.
 11. A sucker rod comprising: (a) an elongate end fitting including: (i) an outer surface portion proximate a first end defining a screw thread; (ii) a second end surface defining an aperture and a radially extending groove; (iii) a rod cavity extending longitudinally toward said first end from said aperture in said second surface and having a cavity surface defining plural frustoconical portions each having a smaller diameter base proximate said second surface and a larger diameter base distal of said second surface; and (b) an insert comprising: (i) a first annular segment having an outer surface substantially conforming to a portion of said cavity surface and an inner surface defining a first portion of a screw thread on a circular arc, said first annular segment having a chord length shorter than a lesser diameter of said aperture and said smaller base; (ii) a second annular segment having an outer surface substantially conforming to a portion of said cavity surface and an inner surface defining a second portion of said screw thread on a circular arc, said second annular segment having a chord length shorter than a lesser diameter of said aperture and said smaller base; (iii) a first key segment having an outer surface substantially conforming to a portion of said cavity surface and an inner surface defining a third portion of said screw thread on a circular arc, said third portion of said screw thread continuous with screw threads defined on said inner surfaces of said first and said second annular segments; and (iv) a second key segment having an outer surface substantially conforming to a portion of said cavity surface and an inner surface defining a fourth portion of said screw thread on a circular arc, said fourth portion of said screw thread continuous with screw threads defined on said inner surfaces of said first and said second annular segments; and (c) an elongate fiberglass rod having a portion of an outer surface defining a screw thread corresponding to said screw thread defined on said inner surfaces of said segments of said insert, said end fitting secured to said fiberglass rod when said segments of said insert are arranged against said cavity surface and said screw thread of said rod is in threaded engagement with said screw thread on said inner surfaces of said first and second annular segments and said first and second key segments of said insert.
 12. The sucker rod of claim 11 wherein said portion of said outer surface defining said screw thread extends from a first end of said rod to a second end of said rod.
 13. The sucker rod of claim 11 wherein said first annular segment, said second annular segment, said first key segment and said second key segment comprise a thermoplastic.
 14. The sucker rod of claim 13 wherein said thermoplastic is reinforced.
 15. The sucker rod of claim 13 wherein said thermoplastic comprises one of polyphenylene sulfide, polyether ether ketone, polyetherimide and polyphthalamide.
 16. The sucker rod of claim 11 further comprising a sheath encircling a portion of said fiberglass rod.
 17. The sucker rod of claim 11 wherein at least one of said first annular segment, said second annular segment, said first key segment and said second key segment comprises a tab extending from said outer surface and engageable with said groove in said second surface of said end fitting. 